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4. Biomechanics of Cornea The analysis, studying ,and reviewing the changing in the balance with the deformation of the corneal tissue under the application of any force defined as the Biomechanics of Cornea 12.The soft tissues, such as the cornea, composed of deferent biochemical and physical nature which determine the main structural and formation of that tissue 13.The main reason we study in details the biomechanics of the cornea is to facilitate forming devices which are able to measure as much as possible the details of the changing and deformation of the cornea . for instance , the corneal elasticity, the static properties, comes from the collagen in Bowman’s layer and storm14. The ground substance, formed mostly be proteoglycans and keratocytes or fibroblasts, would provide the viscous behaviour which refers for the dynamic (time-dependent properties)and arises from the non- covalent rearrangements of the ECM, such as from water diffusion and electrostatic interactions between GAGs and collagen The corneal epithelium accounting for 10% of the central corneal thickness. Be attention that the corneal epithelium is easily deformed and the main reference surface of the biomechanics corneal measurements15. Elastic properties : The collagen fibers are a material which form the Elastic behavior of a tissue . We can get more details about Elastic behavior from the stress strain curve . Basically ,The stress strain curve divided into 3 main zones , first is called toe region which can be happen when applying a light force ( pre stress force) on corneal tissue lead to immediate deformation . The second state is the physiological region which is define as the end of toe region and the beginning elastic region which happens under stress condition and can goes back to normal structural situation 16. Also, the elastic region on the stress strain curve known as Young’s of modulus . The last zone is the plastic region which a corneal tissue can not goes back to normal situation and the rupture of the corneal tissue happen . The cornea tissue consider non -linear elastic material . Measurements of the corneal elastic modulus range from 1.3 MPa – 5 MPa in humans and from 1.5 MPa – 3 MPa in pigs17. Viscoelastic properties : Define as dynamic deformation response which is completey reversible and it is time-dependent behaviour that means when you remove the force applied on the corneal tissue with the time the changes goes back to normal condition . The dynamic modulus E are out of phase in a viscoelastic material, see figure 1d . by calculated be then can modulus dynamic The E i + E0 = E*?? viscoelastic of kind This porcine in that demonstrated characterization dominant was 8kPa)–(2 modulus storage the corneas . kPa) 1.2–(0.3 modulus loss the over is stress the and performed are test relaxation-stress equation: following the to fitted a at stress the ri and stress infinite the is r/ where is e0 strain the that Given si. point time given is properties viscoelastic define to possibility AnotherTypically, series. Prony element-n an use to the relaxation,-stress during stant con maintained easily be can t point time given a at moduli elastic 1c. Figure also See e0. by r(t) dividing by calculated applied been has definition scoelasticvi of kind Thisfrom parameters biomechanical retrieve to study a in .18 deformation puff

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